When checking version, raise errors on django beta version

[giginet]

I tried to run my app with django-permission on django1.7 RC1

However, the error was occurred on following line.

https://github.com/lambdalisue/django-permission/blob/master/src/permission/templatetags/patch.py#L7

It seems to be caused by django.get_version() returns 1.7rc0, so StrictVersion can't recognize this value as version.

Please check it out.